What's the "Sixth Sense"?

A lot of people seriously take this too many ways, like
- Knowing that someone's behind you(MW2 FTW)
- Being able to see the living dead
- Extra visionary
etc.

Can anyone tell me what a somewhat of an exact answer is?

A sixth sense is intuitively knowing something that wasn't input to your brain from your five senses. It's the default explaination for knowing something when you can't explain how you learned it. A glorified "hunch."